The rapid rise your bread gets in the first 10-15 minutes of baking, as trapped gas expands and the yeast have one final burst of activity before dying off around 60C (140F).
Good oven spring is why your bread splits open at the score marks and puffs up dramatically. It requires several things working together: properly fermented dough with enough gas still trapped inside, a hot oven (230C / 446F or higher), adequate steam to keep the crust flexible, and a good score to direct the expansion. Poor oven spring usually means overfermentation (the gas was already spent), insufficient steam, or an oven that wasn't hot enough.
